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Stereochemical aspects of Ziegler-Natta catalysts
Abstract
The advent of MgCl$\sb2$ supported Ti catalysts and more recently the chiral ansa-bridged metallocene homogeneous systems has sparked interest concerning the stereochemical nature of these two classes of Ziegler-Natta catalysts. A classical method of separating chiral sites in heterogeneous systems is by the introduction of an optically active third component and studying the physical properties of the fractionated polymer obtained from the polymerization of racemic monomer. In the homogeneous case an optically pure catalyst may be obtained and as described above, information may be gleaned from racemic monomer polymerization. The system MgCl$\sb2\cdot$3ROH/phthalic anhydride/TiCl$\sb 4$/dialkylphthalate//triethylaluminum/phenyltriethoxysilane was studied. The effect of changing the alkyl group of the ester was that the bulkier alkyl resulted in poorer stereospecific yields of the system. Magic angle spinning cross-polarization $\sp{13}$C NMR revealed that the ester was rigidly bound to the support and that the above system had clearly more than one type of complexation of the ester unlike an earlier supported system indicating more diverse defect structures on the surface. The system was modified with (R,S)- and (S)-2-methyl-1-butanol accompanied by the respective phthalate esters. Activities of 3- and 4-branched $\alpha$-olefins were more depressed than that of earlier supported systems. Active site counting in the polymerization of 4-methyl-1-pentene with a supported system revealed that fewer active sites are available for this monomer than for propylene. Polymerizations of racemic monomer were stereoelective but for the 3-branched monomer the polymer produced was more stereoirregular than those from earlier supported systems. Polymerization of racemic 4-methyl-1-hexene with the homogeneous (S)-ethylenebis($\eta\sp 5$-tetrahydroindenyl)zirconiumbis(O-acetyl-(R)-mandalate)/methylaluminumoxane system produced more optically pure and stereoregular polymer than the heterogeneous systems. The existence of fractions in the polymer of opposite optical rotation suggests that at least two stereochemical environments exist during the course of the polymerization, disputing earlier beliefs that this system is stereochemically rigid. Also, it was found that the type of solvent used in the polymerization could affect the stereochemical distribution of the system.
Subject Area
Polymer chemistry|Organic chemistry
Recommended Citation
Vizzini, James Charles, "Stereochemical aspects of Ziegler-Natta catalysts" (1991). Doctoral Dissertations Available from Proquest. AAI9207467.
https://scholarworks.umass.edu/dissertations/AAI9207467